Thermal transfer printing

ABSTRACT

An article for receiving an image by thermal transfer printing includes an image-receiving surface of plastics material coated with a layer of dye-receptive material capable of accepting an image by means of a dye thermal transfer process, wherein the glass transition temperature (Tg) of the dye receptive material is at least 50° C. lower than the softening point of the plastics material of the image-receiving surface. By having a difference between the softening point of the plastics material article surface and the Tg of the material coated thereon of at least 50° C. it is possible to achieve printing in a single step transfer process of high density images without distortion of the plastics material. The invention can be used with three-dimensional articles possibly having compound curves, including casings for mobile telephones made of polymers such as polycarbonate.

FIELD OF THE INVENTION

This invention relates to thermal transfer printing, and particularlyconcerns an article for receiving an image by thermal transfer printing,a method of printing and an article bearing a printed image.

BACKGROUND TO THE INVENTION

Thermal transfer printing, in the context of the present specification,is used to mean a process in which an image on a carrier sheet isprinted onto an image-receiving surface of an article by placing theimage in contact with said surface of the article, and heating the imageto transfer the image onto the article surface. Usually force is appliedto exert pressure on the sheet and maintain the sheet in contact withthe surface during printing.

The image is typically formed on the carrier sheet by a first printingstep. In this case, the overall two stage process can be considered as aretransfer printing process. Retransfer printing techniques are commonlyused for printing on articles other than flexible sheet material.

One example of a retransfer printing process is dye diffusion thermalretransfer printing. In a first stage, an image is formed by dyediffusion thermal transfer printing on a retransfer intermediate carriersheet. Dye diffusion thermal transfer printing is a process in which oneor more thermally transferable dyes are caused to transfer from selectedareas of a dye-donor sheet to a receiver by thermal stimuli, thereby toform an image. This is generally carried out in a printer having athermal head or laser energy source, depending on the kind of dye-donorsheet used. Using a dye-donor sheet comprising a thin substratesupporting a dyecoat containing one or more uniformly spread dyes,printing is effected by heating selected discrete areas of the dye-donorsheet while the dyecoat is pressed against a dye-receptive surface of areceiver sheet, thereby causing dye to transfer to corresponding areasof the receiver. The shape of the image transferred is determined by thenumber and locations of the discrete areas which are subjected toheating. Full colour images can be produced by printing with differentcoloured dyecoats sequentially in like manner, and the differentcoloured dyecoats are usually provided as discrete uniform panelsarranged in a repeated sequence along a ribbon-shaped dye-donor sheet orink ribbon. In retransfer printing, the receiver sheet is in the form ofa retransfer intermediate sheet which comprises a supporting substratehaving a dye-receptive imageable layer on one side, usually with abackcoat on the other side to promote good transport through the initialprinter. Suitable retransfer intermediate sheets are disclosed, e.g, inWO 98/02315 (EP 912349). The image-carrying intermediate sheet formed inthe first stage of the process is separated from the dye-donor sheet,and in the second stage of the process, is pressed against the article,with its image-containing layer contacting a dye-receptive surface ofthe article. Heat is then applied to effect transfer of the image,usually over the whole area of the image simultaneously. This iscommonly carried out in a press shaped to accommodate the article. See,for example, the mug press disclosed in U.S. Pat. No. 5,643,387.

Thermal retransfer techniques are of particular applicability to thecustomisation of three dimensional articles such as mugs and tiles madeof materials of high thermal stability that can withstand thetemperatures involved without distortion. The carrier sheet can beprinted by a desktop printer, and the press can retransfer the image inless than 5 minutes. Thus the method has advantages over direct printingby silk-screen, offset or gravure processes because it can addpersonalisation at point-of-sale.

In general, prior art thermal retransfer techniques can only be usedwith articles made of materials able to withstand the elevatedtemperatures (and possibly also elevated pressures) involved withoutdistorting or breaking, typically being restricted to articles of metalor refractory materials such as ceramics. This applies, e.g., to the mugpress disclosed in U.S. Pat. No. 5,643,387, which employs high pressuresto ensure compliance of the press with the surface of the mug to beprinted, restricting the approach to refractory objects of certain forms(having only simple curves, in only one plane) which do not distort orbreak at the temperatures and pressures involved.

The specification of British Patent Application No 0100330.0 filed6^(th) Jan. 2001 (pursued as PCT/GB02/00037) discloses apparatus forprinting an image onto an image-receiving surface of an article by athermal transfer printing process, the apparatus comprising aheat-conductive member comprising foamed polymeric elastomer materialand having a surface shaped to be substantially complementary to theimage-receiving surface of the article; and means for heating themember.

In use, a carrier sheet of material bearing an image to be printed isheld in contact with the image-receiving surface of the article by themember, via the complementary surface of the member. Typically, a sheetis located on the article in an appropriate position and the member isbrought into contact with the exposed face of the sheet and forceapplied to exert pressure on the sheet sufficient to promote wettingcontact between the sheet and the member. The member is heated by theheating means, possibly being pre-heated to a desired temperature priorto contact with the sheet and/or prior to contact of the sheet with thearticle. The sheet is held in contact with the article for a suitabletime with the member at a suitable temperature to print the image on thesurface of the article. Suitable printing conditions can be readilydetermined by experiment. The member is removed and the article allowedor caused to cool to ambient temperature. The sheet is usually removedfrom the article surface.

Because heat is applied via the member, rather than being applied viathe article, the article is exposed to less heat. The apparatustherefore enables printing to be performed on articles of a wider rangeof materials than had been possible hitherto. In particular, it ispossible to print on articles of thermoplastic materials such as manywidely used polymers such as acetal resin, acrylic resins,polycarbonates etc, provided the article is suitably supported whenheated so the shape of the article is retained.

The apparatus accordingly desirably includes a support for the article,shaped to support or receive the article while leaving exposed theimage-receiving surface thereof. In embodiments intended for use inprinting articles of thermoplastic material, the support should beshaped to be complementary to a part of the article so as fully tosupport the article with a snug fit. The support will be discussedfurther below.

During printing, the sheet should be held firmly in contact with thearticle to produce a print of good quality. The apparatus thereforepreferably includes suitable urging means, eg for applying force betweenthe member and support, e.g. in the form of a spring loading arrangementor a hydraulic system.

The member comprises foamed material, ie. material including a pluralityof small openings, pockets or voids within the body of the material. Thevoids are typically filled with gas, commonly air. The voids should besmall (preferably having a maximum dimension less than 1 mm) and areideally substantially uniform in size and are preferably reasonablyuniformly dispersed through the material, for uniformity of properties,such that any variation in conductivity is not such as to produce printswith a variation in optical density. The presence of the voids, coupledwith the elastomeric nature of the member material, mean that the memberis relatively soft and has a degree of elasticity and so can, ifnecessary, deform to an extent (at least when heated) to enable thesurface thereof to conform to the image-receiving surface of an article,thus providing uniform contact between the member and article. Providedthe member is sufficiently compliant to allow good wetting contact withthe sheet and article under compression, variation between the shape ofthe member and article surfaces is tolerable. Thus, while said surfaceof the member should be shaped to be substantially complementary to theimage-receiving surface of the article, exact complementarity is notrequired as a degree of variation can be accommodated by thedeformability of the member. The degree of variation tolerable willdepend on factors including the degree of deformability of the member,the pressure applied in use and the complexity of shape of theimage-receiving surface of the article. References to “substantiallycomplementary” in this context should be construed accordingly.

The void volume of the material should be selected to give desiredproperties, and should be sufficiently high to provide the necessaryelasticity without being so high as to adversely affect thermalconductivity. Generally, the void volume is in the range of 5 to 35% byvolume, typically being about 10% by volume.

Suitable foamed polymeric elastomer materials are well known to thoseskilled in the art, and are commercially available or can be produced byknown techniques. A foamed material can be created with a controlledvoid level by several means, including but not limited to mechanicalentrainment of gas, adding volatile liquids, dissolving gases in one ormore components, using a chemical reaction which evolves gas, adding amaterial which decomposes to evolve a gas during curing. For example,air-filled voids can be created by vigorously mixing material with airto generate an aerated paste which sets, eg on addition of a curingagent, to produce a solid foamed material.

The foamed polymeric elastomer material preferably comprises a foamedsilicone resin, but other foamed polymers that exhibit sufficientflexibility under printing conditions may alternatively be employed.Suitable materials are commercially available.

The member comprising foamed polymeric elastomer material is heatconductive so that in use heat can be conducted through the member to animage-bearing sheet in contact with the article. It is desirable for themember to have relatively high thermal conductivity properties so thatin use the sheet reaches the transfer temperature rapidly, thus avoidingthe need for prolonged exposure of the article to elevated temperaturesand so reducing the likelihood of thermal degradation of the article.The member preferably has a thermal conductivity of at least 0.2 W/m/K,and in general desirably has as high a thermal conductivity as possibleprovided other properties are not compromised.

The presence of voids in the polymeric elastomer material reduces thethermal conductivity of the material. In order to raise the thermalconductivity, preferably to a value greater than that of the polymericelastomer material in unvoided, unfoamed condition (eg unfoamed siliconeresin), it is convenient to include in the material particles of highthermal conductivity material, having a thermal conductivity greaterthan that of the polymeric elastomer material and typically having athermal conductivity of at least 1 W/m/K. Suitable high thermalconductivity materials include metals and metal oxides. The particlesshould be reasonably uniformly dispersed through the material, foruniformity of properties. The particles are preferably spherical or nearspherical in form and relatively uniform in size in order not to affectadversely the flexibility and deformability of the member. For the samereason, the particles are preferably relatively small, eg having amaximum particle size in the range 3 to 30 microns. The particles alsodesirably have a maximum dimension less than 0.2 mm to avoid the risk ofsurface roughness of the member that might deform and emboss the articlein use. The particles are desirably present in sufficient quantity toensure that the image reaches and stays at the transfer temperature forprinting for the required period of time, without being present at suchhigh levels that flexibility of the material is compromised. Suitableparticle loadings can be readily determined by experiment. The particlesare preferably present at a level of at least about 10% of the volume,more preferably at least about 15% or more of the volume of thematerial.

The member is preferably made of material that is substantiallyisotropic, having voids, and particles if present, reasonably uniformlydistributed therethrough.

The means for heating the member conveniently comprises a heatable plateor jacket in contact with the member other than said shaped surfacethereof. The plate or jacket may be heatable in known manner, egincorporating or being linked to electrically operated heating elements.

Alternatively or additionally, the member be rendered electricallyconductive by the incorporation in the foamed material of particles ofelectrically conductive material, preferably carbon, so that the membermay be heated by the passage of an electrical current therethrough.

The apparatus preferably includes a support, as noted above.

The support preferably has a lower thermal conductivity than that of themember, eg being of the same base material as the member but not infoamed condition and without added particles of high thermalconductivity material. For instance, the support conveniently comprisesunfoamed silicone resin. This is because, in use, heat flow through anarticle tends towards a steady-state value during printing. Unless thethermal conductivity of the support is less than that of the foam, thesupport will suck heat away from the article, making it harder for thearticle to attain a suitable temperature for printing and necessitatingthe use of higher printing temperatures, which is generally undesirable.It is nevertheless possible to produce satisfactory prints usingapparatus in which the support has higher thermal conductivity than thatof the member. At least in embodiments intended for use in printing onarticles of thermoplastic materials, the support should be comparativelyhard, rigid and non-deformable.

The voids in the foamed material serve to make the material morecompliant and deformable. Hence the voids are used in theheat-conductive member on the printing side in order to allow goodconformation between the member and the combination of carrier sheet andarticle to be printed. The voids have the undesirable side effect ofreducing the thermal conductivity of the material. This effect is morethan compensated by adding thermally conductive filler material to thecomposition. In the case of the support, the material should not be toosoft and deformable, or it will not be capable of providing the desiredmechanical support. For this reason, it is preferred that there shouldbe no voids present. It is also desired that the thermal conductivityshould be lower than that of the other member. The absence of voids is adisadvantage from this point of view. However, the desired result can beachieved by not incorporating the thermally conductive filler in thesupport.

The support is preferably not heated during printing, to reduce exposureof the article to heat, and also for more rapid cooling of the articleafter printing.

The apparatus may include cooling means for cooling the article afterprinting and after removal of the member. For instance, means such as afan may be provided for directing a flow of cold air over the printedsurface of the article. Additionally or alternatively, the support mayhave an associated coolable member.

The support is typically vertically below the member in use of theapparatus, but this is not essential.

The apparatus is typically designed for use with a particular article,with the member, and support if present, being tailored to the exactshape of that particular article.

The specification of International Patent Application No. PCT/GB02/00037also discloses a method of printing an image on an image-receivingsurface of an article, comprising holding a carrier sheet bearing animage to be printed in contact with said image-receiving surface of thearticle by means of a heat-conductive member comprising foamed polymericelastomer material and having a surface shaped to be substantiallycomplementary to said image-receiving surface of the article; andheating the member to print the image on said surface.

The image on the carrier sheet may be produced by a variety of differentprinting techniques, including dye diffusion thermal transfer printing,using appropriate carrier sheet material.

For example, a dye diffusion thermal retransfer printing process using aretransfer intermediate sheet as discussed above may be adopted. Aretransfer intermediate sheet typically comprises a supporting substratehaving a dye-receptive coating, or receiver layer, on one surfacethereof, e.g. as described in EP 409514, with the other surfaceoptionally carrying a coating to improve the friction, release or staticproperties of the sheet during printing and preferably also carrying theheat-resistant backcoat, as described in EP 912349 (WO98/02315). Thesubstrate may comprise paper, possibly polyolefin-coated paper, with thepreferred substrate comprising a laminated material prepared bylaminating an opaque, voided polypropylene film to at least one side ofa cellulose paper base material, e.g. as disclosed in JP 06-84119-B. Thevoids in the polypropylene film layer improve the compliance of thesheet.

The substrate is preferably not too stiff or inelastic, at least atprinting temperatures and preferably also at ambient temperature, atleast for use in printing on a compound curved surface (concave orconvex) of an article, such as a part spherical surface having curvaturein two planes, to enable the sheet to conform to the surface withoutcreasing. It may be desirable to pre-heat the sheet prior to holding thesheet in contact with the article so that the sheet is in softenedcondition and more readily able to conform to the shape of the surface.Such pre-heating can be achieved by radiant heat from the member inheated condition, being held in close proximity to the sheet located onthe image-receiving surface of the article (but not held in contacttherewith), prior to contact of the sheet by the member and applicationof force to press the sheet into engagement with the article.

The image-receiving surface of the article may optionally be coated withsuitable material to enhance susceptibility of the surface to printing,in known manner. Examples of suitable coating materials are given inU.S. Pat. No. 5,643,387 at column 2 lines 23 to 52. U.S. Pat. No.4,943,684 and EP 721848 also disclose coating materials suitable for useon ceramic articles.

The method and apparatus disclosed in PCT/GB02/00037 are thus applicableto printing on articles of a wide range of materials, includingthermoplastic materials as well as metals and ceramic materials, and ina wide range of shapes, including printing on flat surfaces as well ason more complex, curved shapes (concave or convex) including compoundcurves. This approach can thus be used for printing on articles such ascasings of mobile telephones made of polymers such as polycarbonate thatsoften and would otherwise distort under printing conditions oftemperature and pressure. With such articles even slight distortion isunacceptable because the article must meet tight tolerances to fitcorrectly with other components.

Thermal transfer printing of an image onto an article of plasticsmaterial, as compared with an article of a high thermal stabilitymaterial such as ceramic or metal, is a much more difficult, sensitiveprocess, requiring better temperature control in the article to avoidunwanted distortions resulting from flow of the plastic. Conversely, itis necessary to use relatively high transfer temperatures in order toproduce good quality high density colour images. It is known to use atwo stage process for thermal transfer of an image onto an article ofplastics material, with the first stage involving dye transfer bysublimation and the second stage involving dye fixation by use of rapidinfra-red heating: see FR 2728505.

JP 3132862 discloses a mass retransfer system in which an image on acarrier sheet is mass transferred to an article of plastics material.The article bears an image-receiving layer of lower softening point thanthat of the plastics material. The image-receiving layer is designed toprovide high adhesion with the material to be printed, and so isunsuitable for receiving an image by a dye thermal transfer processwhere efficient release of the carrier and image-receiving layer isrequired.

SUMMARY OF THE INVENTION

One aspect of the invention provides an article for receiving an imageby thermal transfer printing, the article including an image-receivingsurface of plastics material coated with a layer of dye-receptivematerial capable of accepting an image by means of a dye thermaltransfer process, wherein the glass transition temperature (Tg) of thedye-receptive material is at least 50° C. lower than the softening pointof the plastics material of the image-receiving surface.

An image can be printed on the image-receiving surface of the article bya dye diffusion thermal transfer process as described above in relationto PCT/GB02/00027, conveniently using apparatus as described inPCT/GB02/00037 including a support for the article. In this case, acarrier sheet of material bearing an image to be printed is held incontact with the image-receiving surface of the article by the member,via the complementary surface of the member. Typically, a sheet islocated on the article in an appropriate position and the member isbrought into contact with the exposed face of the sheet and forceapplied to exert pressure on the sheet sufficient to promote wettingcontact between the sheet and the member. The member is heated by theheating means, possibly being pre-heated to a desired temperature priorto contact with the sheet and/or prior to contact of the sheet with thearticle. The sheet is held in contact with the article for a suitabletime with the member at a suitable temperature to print the image on thesurface of the article. Suitable printing conditions can be readilydetermined by experiment. The member is removed and the article allowedor caused to cool to ambient temperature. The sheet is usually removedfrom the article surface.

By having a difference between the softening point (which is also knownas the mould temperature) of the plastics material of the articlesurface and the Tg of the material coated thereon of at least 50° C.,preferably at least 60° C., more preferably at least 65° C., theinventors have found it possible to achieve printing in a single steptransfer process of high density images without distortion of theplastics material. As noted above, pressure is generally used to obtainwetting contact between the carrier sheet and the dye-receptive coatingon the article. In order to prevent distortion of the plastics materialof the article during such a transfer process it is necessary for thetemperature of plastics material not to exceed the softening point ofthe material. This requirement therefore constrains the temperature atwhich thermal transfer of the image can be carried out in a way thatwould normally result in loss of optical density of the image ontransfer. While such loss of optical density may be overcome byincreasing the transfer time, this in turn results in a reduction inimage sharpness and quality. However, because the Tg of thedye-receptive coating is substantially lower (by at least 50° C.) thanthe softening point of the plastics material, in accordance with theinvention, this enables the transfer process to be carried out at atemperature that results in good transfer optical densities withoutexceeding the softening point of the plastics material (and so notaffecting the integrity of the article) and without compromising imagesharpness and quality.

The dye-receptive coating material should be selected having regard tofactors including the softening point of the plastics material of theimage-receiving surface of the article.

The dye-receptive coating material should have a Tg of at least 30° C.in order to produce an image of adequate stability on the printedarticle under normal conditions of use: if the Tg is too low, the dye islikely to migrate in the material, affecting image quality. Thedye-receptive coating preferably has a Tg of at least 45° C. A coatingwith a Tg in the range 45-50° C. generally provides good imagestability; furthermore, there are available a wide range of plasticsmaterials having a softening point at least 50° C. higher than thisrange and so suitable for use as the image-receiving surface of thearticle. Suitable coating materials can be readily selected by oneskilled in the art, and include polyesters, acrylic materials, polyvinylchloride (PVC) etc. The coating material preferably comprises across-linked polymer, e.g. cross-linked with a mould releasing agent tobe discussed below. The polymer is preferably lightly cross-linked: ifthe level of cross-linking is too high dye penetration is inhibitedthereby reducing print densities, whereas if the level of cross-linkingis too low the coating is liable to flow and stick to a carrier sheet inuse. The molecular weight of units between cross-linkable functionalgroups of the polymer is preferably greater than 900 and less than10,000 to confer a suitable level of light cross-linking. Suitablecoating materials are commercially available

The dye-receptive coating material may be mixed with a suitable solventor thinner to give an appropriate consistency.

The dye-receptive coating preferably includes one or more suitable mouldreleasing agents, such as silicones, fluorocarbons and alkyl resins.Suitable materials are commercially available and are known to thoseskilled in the art. Such agents function to prevent sticking of acarrier sheet to the image-receiving surface. Ideally, the mouldreleasing agent is cross-linked with a polymeric dye-receptive coatingmaterial, as discussed above.

The dye-receptive coating material may be applied to the image-receivingsurface of the article in any convenient way, as will be known to thoseskilled in the art, eg by spraying, painting, dip coating, spin coating,screen printing etc., usually followed by drying and curing. Spraying isthe currently preferred technique.

The dye-receptive coating conveniently has a thickness in the range 10to 50 microns.

At least the image-receiving surface of the article is of plasticsmaterial, but in a typical case all of the article is of the plasticsmaterial. The plastics material is typically a thermoplastic material,and suitable plastics materials having appropriate softening pointsinclude ABS (a ter polymer composed of three different monomers,Acrylonitrile: Butadiene: Styrene), blends of ABS with other polymerse.g. polycarbonate (ABS/PC), acrylic resins, polycarbonates,polysulphones, polyethersulphones (PES), polyether ether ketones (PEEK)etc.

The article may have a wide variety of forms and shapes, includingthree-dimensional articles possibly having complex, curved shapes(concave or convex) including compound curves. The invention findsapplication, for instance, in printing of articles such as casings ofmobile telephones made of polymers such as polycarbonate.

Printing on an article in accordance with the invention is convenientlycarried out using a dye diffusion thermal transfer method (usually aretransfer method) as disclosed in GB 0100330.0, as described above,e.g. using carrier sheets (usually retransfer intermediate sheets) andapparatus as described above.

In a further aspect, the invention thus provides a method of printing animage by dye thermal transfer on an image-receiving surface of anarticle in accordance with the invention, comprising holding a carriersheet bearing an image to be printed in contact with said surface, andheating the carrier sheet to print the image on said surface.

The carrier sheet is heated to a temperature above the Tg of the coatingbut not exceeding the softening point of the plastics material.

The invention also includes within its scope an article bearing aprinted image produced by this method.

The invention will be further described by way of illustration, in thefollowing examples which used apparatus as described in PCT/GB02/00037,as illustrated in the accompanying drawings, in which:

FIG. 1 is a schematic sectional view of printing apparatus; and

FIG. 2 is a view similar to FIG. 1, showing the apparatus in use.

DETAILED DESCRIPTION OF THE DRAWINGS

Referring to the drawings, the illustrated apparatus comprises aprinting press designed to print an image onto a surface of a plasticsarticle 10 moulded from high temperature ABS/PC polymer (Bayblend T45 MN(Bayblend T45 MN is a Trade Mark) manufactured by Bayer AG) having asoftening point of 110° C. by a dye diffusion thermal retransferprinting process.

The apparatus comprising a support 12 of silicone resin, which is aprecisely moulded part shaped to fit exactly within the article 10. Thesupport is mounted on a coolable backing plate 14.

The apparatus further comprises a heatable aluminium element 16, havingthe general form of an inverted open box. Element 16 has associatedelectric heaters (not shown) either inserted therein or attached to themajor outer face thereof. Element 16 carries a rubber-like foam member18, the inner surface 20 of which is shaped to be complementary to theouter face 22 of the article 10. Member 18 is formed of foamed siliconeresin containing both gas-filled voids and thermally conductiveparticles.

In use, the article 10 is fitted on support 12. A retransferintermediate carrier sheet 24, bearing an image to be printed on oneface 26, produced by dye diffusion thermal transfer printing asdiscussed above, is located on the outer face 22 of the casing, with theimage positioned over the region of the article on which it is desiredto print. Element 16 is heated, thus heating member 18 by conduction.The member 18 is pre-heated in this way until it is at a temperatureabove the retransfer printing temperature, typically being up to about200° C. Heat radiating from the surface 20 of member 18 will heat up,and may cause softening of, sheet 24. The member 18 (and associatedheater 16) are moved into contact with the sheet 24 and exposed regionsof the article outer face 22, to the position shown in FIG. 2, andsuitable force applied by a spring loaded arrangement (not shown) topress the sheet 24 into engagement with the surface 22 of the article.In this position, the article 10 is fully enclosed between support 12and member 18. This condition is maintained for a suitable time,typically about 1 or 2 minutes. During this time the sheet and image andalso the article are rapidly heated to a temperature at which dyediffuses to the article surface, creating a durable image. The entireapparatus may optionally be located in a suitable enclosure and airevacuated to ensure good contact between the sheet and article duringprinting.

Any air trapped between the article and the sheet will inhibit thetransfer of the dye on printing and give rise to unprinted spots on thearticle. This can be avoided by extracting the air from the press priorto closing it or by arranging the shape of the foam to squeeze the airout progressively from the point of contact to the edges as the press iscooled. This can be achieved, e.g., by having a slight dome (not shown)in the centre of member surface 20 so that as pressure is progressivelyapplied an outwardly enlarging area of contact between the surface 20and the sheet 24 and article face 22 is progressively formed, movingfrom the centre to the edges and forcing out air.

The foam of member 18 is soft enough to conform closely to the surface22 of the article, deforming to accommodate any differences in shape,and so ensuring uniform contact. The voids provide sufficient elasticityto maintain an even pressure despite dimensional variations betweencomponents. The particles give sufficient heat capacity and thermalconductivity to ensure that the image reaches and stays at theretransfer temperature for the required period of time. The support isshaped to prevent the article deforming even though the retransfer maytake place at a temperature above the softening temperature of thearticle material.

After printing, the press is opened by raising the member 18 and element16, leaving the article 10 and sheet 24 on the support 12. The thermalconductivity of the support is such as to allow the casing to coolquickly to below its softening temperature. Cooling may be assisted bydirecting cold air over the casing. After cooling, the article 10 can beremoved from the support without risk of distortion of the article. Thesheet 24 can be peeled off the casing, leaving a printed image on thesurface 22.

Member 18 is made of a silicone resin foam containing 40% w/w ofaluminium powder which was prepared as follows: 20 grams of Silastic ‘S’silicone base (Dow Corning) (Silastic ‘S’ is a Trade Mark) were mixedvigorously with 13.3 grams of fine aluminium powder of particle size3-30 microns (Fisher Chemicals A/1605/53) to form a finely aeratedpaste. Then 2 grams of Silastic ‘S’ curing agent were mixed in. Theresulting paste was cast into a slab and cured at 100° C. for 3 minutes.The volume fraction of aluminium powder is limited by the thickening ofthe paste and the need to retain flexibility in the cured resin. Tilevolume composition of the foam material is 71% resin, 18% metal, 11%voids.

Support 12 was made of the same silicone resin, but in non-voided formand without added aluminium powder.

EXAMPLES Example 1

The ABS/PC polymer articles 10 was coated (by spraying) with thefollowing dye-receptive clear coating formulations.

1. PPG Clearcoat (PPG Clearcoat is a Trade Mark) paint lacquer having aTg of 67° C. comprising resin 84-46-0008 (10 g), cross-linking hardeningagent 87010002 (5 g), and thinner 89060001 (5 g). The Clearcoat lacquerwas made from the 3 components in accordance with the manufacturersinstructions.

The softening point of the ABS/PC (110° C.) was 43° C. higher than theTg of the PPG Clearcoat paint lacquer. This coated article is not inaccordance with the invention, and is included for comparative purposes.

2. A coated article in accordance with the invention was made using acoating having the following formulation:

1. Dynapol LH833 (50%) 1.5 kg 2. Silwet L77 10.00 g 3. Additol XL1237.95 g 4. Tegomer 2711 1.98 g 5. Uvitex OB 1.30 g 6. Tinuvin 144 7.90 g8. DABCO T12 0.50 g 9. Butyl Acetate 583.0 g

Dynapol LH833, Silwet L77, Additol XL123, Tegomer 2711, Uvitex OB,Tinuvin 144 and DABCO T12 are all Trade Marks. Dynapol LH833 is apolyester, supplied as a 50% by weight polymer solution in solvesso 150solvent. Silwet L77 is an alkoxy silicone used to give very goodsubstrate wetting. Additol XL123 is a silicone based flow agent toprevent surface defects. Tegomer 2711 is a hydroxy end functional polydimethyl siloxane release agent. Uvitex OB is an optical brightener toensure the coating appears white. Dabco T12 is an isocyanatecross-linking catalyst. Truvin 144 is a hindered amine to prevent uvdegradation of the polyurethane coating. Butyl acetate is a solvent.

The ingredients were mixed together with 200 g of the PPG isocyanatecross-linking agent 87010002 as used in formulation 1 (i.e. at a weightratio of 10.6:1 coating ingredients:cross-linking agent) to produce across-linked coating having a Tg of 48° C., i.e. 62° C. lower than thesoftening point of the ABS/PC of the articles 10.

The coated articles were oven cured for 30 minutes at 80° C. and left tostand for one week before being subject to a retransfer step. There-transfer step was carried out using the re-transfer apparatusdescribed above with reference to FIGS. 1 and 2, using an image on aretransfer intermediate sheet of VP retransfer paper from ICI Imagedata.The retransfer paper comprises a 128 gsm paper core laminated on bothsides with a 35 microns thick commercial pearl film such as Toyopearl SS(Toyopearl SS is a Trade Mark). The upper layer of the substrate iscoated with a filled whitening layer upon which the receiver layer iscoated. The member 18 was heated to a temperature of 200° C., giving aninterfacial temperature of less than 110° C. When the press was closed,the article 10 was subjected to a pressure of 500 Newtons for 90seconds. The press was then opened and the article left on the supportto cool, with a flow of air being directed over the support and shellfor 30 seconds for additional cooling effect.

A full colour (black, cyan, magenta, yellow) image was transferred intothe coatings on the articles in this way. The neutral optical density ofthe images in the coating was measured with a Macbeth TR-1224densitometer in reflection mode, and the article checked for distortion.The results are set out in the Table below.

Coating Yellow Magenta Cyan Black Distortion 1 0.88 0.85 0.4  0.82 N 22.16 1.67 0.92 2.04 N

The coated article in accordance with the invention (coating 2) clearlygives images with higher dye densities than those obtained using coating1. When further tests were carried out printing on coating 1 at higherretransfer temperatures the plastic article distorted.

Example 2

A series of further coatings were prepared, generally similar to coating2 of Example 1, using Dynapol LH833 as before and also Vylon GK130(Vylon GK130 is a Trade Mark) which is a low molecular weight, low Tg,hydroxyl functional linear polyester. Coating formulations were asfollows:

1. Polymer 0.75 kg 2. Silwet L77 10.00 g 3. Additol XL123 7.95 g 4.Tegomer 2711 1.98 g 5. Uvitex OB 1.30 g 6. Tinuvin 144 7.90 g 8. DABCOT12 0.50 g 9. Butyl Acetate 583.0 g

The resin systems were mixed together with PPG isocyanate crosslinkingagent 87010002 as used in the previous example to produce cross-linkedcoatings. Further ABS/PC polymer articles 10 (as used in Example 1) werecoated with the cross-linked coating formulations, using a dip coatingprocess in place of spray coating as in Example 1. The coated articleswere cured at 90° C. for 50 minutes before being printed by a retransferprinting operation, as described in Example 1.

The dye retransfer optical densities were measured using a Gretag SPM50spectrophotometer (Gretag SPM50 is a Trade Mark) and the articles werechecked for any distortion. Results were as follows:

Resin: Glass Retransfer Resin Crosslinker Transition optical densitysystem ratio Tg ° C. y m c k Dynapol LH833 10:1 48 1.95 1.87 1.18 1.96Dynapol LH833 + 10:1 48 1.82 1.75 1.14 1.91 30% w/w TiO₂ filler VylonGK130 10:1 30 2.19 2.07 1.37 2.20

All the samples exhibited a Tg difference of greater than 50° C.compared with the plastic part. All the samples gave good retransferdensities.

It should be noted that the test pattern used to print the above sampleswas different to that used in Example 1. The above testing was carriedout with maximum density Y, M, C and K bars whereas the earlier testsamples used full power Y & K, but lower power M and C. Coating 2 ofExample 1 is identical to the first entry in the table above, and the Yand K density data for the two coatings show reasonably good agreement.

1. An article for receiving an image by thermal transfer printing, thearticle including an image-receiving surface of plastics material coatedwith a layer of dye-receptive material capable of accepting an image bymeans of a dye thermal transfer process, wherein the glass transitiontemperature (Tg) of the dye-receptive material is at least 50° C. lowerthan the softening point of the plastics material of the image-receivingsurface.
 2. An article according to claim 1, wherein the Tg of thedye-receptive material is at least 60° C. lower than the softening pointof the plastics material of the article surface.
 3. An article accordingto claim 1 or 2, wherein the dye-receptive coating material has a Tg ofat least 30° C.
 4. An article according to claim 3, wherein the coatingmaterial has a Tg in the range 45-50° C.
 5. An article according toclaim 1, wherein the dye-receptive coating material is selected from:polyesters, acrylic materials and polyvinyl chloride.
 6. An articleaccording to claim 1, wherein the dye-receptive coating materialcomprises a cross-linked polymer.
 7. An article according to claim 6,wherein the degree of cross-linking is such that the molecular weight ofunits between cross-linkable functional groups of the polymer is greaterthan 900 and less than 10,000.
 8. An article according to claim 1,wherein the dye-receptive coating includes one or more mould releasingagents.
 9. An article according to claim 8, wherein the mould releasingagent is cross-linked with a polymeric dye-receptive coating material.10. An article according to claim 1, wherein the dye-receptive coatinghas a thickness in the range 10 to 50 microns.
 11. An article accordingto claim 1, wherein the plastics material of the image-receiving surfaceis a thermoplastic material.
 12. An article according to claim 11,wherein the plastics material is selected from: ABS, blends of ABS withother polymers, acrylic resins, polycarbonates, polysulphones,polyethersulphones and polyether ether ketones.
 13. An article accordingto claim 1, wherein the article is a three-dimensional article: possiblyhaving complex, curved shapes (concave or convex) including compoundcurves.
 14. An article according to claim 1, comprising a casing of amobile telephone.
 15. A method of printing an image by dye thermaltransfer on an image-receiving surface of an article in accordance withclaim 1, comprising holding a carrier sheet bearing an image to beprinted in contact with said surface, and heating the carrier sheet toprint the image on said surface.
 16. An article bearing a printed imageproduced by the method of claim
 15. 17. An article according to claim 2wherein the Tg of the dye-receptive material is at least 65° C. lowerthat the softening point of the plastics material of the articlesurface.
 18. An article according to claim 3 wherein the dye-receptivecoating material has a Tg of at least 45° C.